1. Field of Invention
This invention relates to semiconductor integrated circuits and in particular to pump circuits used with semiconductor memory devices.
2. Description of Related Art
Pump circuits have become an important function in semiconductor memories for providing internal voltages higher than the voltage applied to the memory chip. The higher internal voltages created by pump circuits are used with specific functions that cannot be designed to operate at the lower chip voltage. This allows a low voltage to be applied to the chip for the use with most functions to lowers the power dissipation and allow higher levels of integration. One of the more critical applications of a pump circuit is to provide a bias voltage for word line drive circuits. The higher voltage for the wordline drive circuit is necessary to enhance the reading and writing current from the memory cell. The pump circuits generally in use include a booster circuit to produce the higher voltage and some form of a pass gate to connect the charge on capacitors of the booster circuit to an output load including an integrating capacitance to help smooth out the resulting voltage.
In U.S. Pat. No. 5,222,042 (Ichiguchi) a boost circuit for DRAM wordline is shown which boosts the wordline signal and minimizes the effect of capacitor leakage by using two boost circuits. In U.S. Pat. No. 5,134,317 (Otah) a charge pump circuit for a DRAM is disclosed in which the time to charge the boost capacitor is reduced. In U.S. Pat. No. 4,673,829 (Gupta) discloses a charge pump for use in programming a memory array that minimizes leakage current for unselected cells.
In "A High Efficiency CMOS Voltage Doubler" by Pierre Favrat, IEEE Journal of Solid State Circuits, Vol. 33, No. 3, March 1998, a voltage doubler is discussed using a charge pump cell and improved serial switches. An fully integrated charge pump is shown with an efficiency of seventy five percent whereas efficiencies of up to ninety five percent were attained using external capacitors. In "An Experimental 1.5-V 64-Mb DRAM" by Nakagome et al., IEEE Journal of Solid State Circuits, Vol. 26, No. 4, April 1991, low voltage circuit technologies for high density DRAM's is discussed including a wordline driver with charge pump circuit achieving a high boost ratio.
One of the problems with pump circuits is the threshold voltage of the pass gate transistor which reduces the amount of charge that can be transferred to the output of the pump circuit. A pass gate transistor that is not fully turned on has a high threshold voltage. Solving this problem can lead to driving the gate of the pass gate transistor to a high voltage, 3Vcc, and can lead to breakdown problems in the circuitry driving the gate of the pass gate transistor. Other approaches that try to eliminate the high gate voltage have difficulties keeping the pass gate transistors fully turned on and provide a low conductance for transferring charge to the output from a booster circuit. There is also a possibility of latch up because a well bias cannot be maintained to the highest voltage which potentially leads to a circuit latch up.